A router has two full-duplex Ethernet interfaces each operating at 100 Mb/s.…
2006
A router has two full-duplex Ethernet interfaces each operating at 100 Mb/s. Ethernet frames are at least 84 bytes long (including the Preamble and the Inter-Packet-Gap). The maximum packet processing time at the router for wirespeed forwarding to be possible is (in microseconds)
- A.
0.01
- B.
3.36
- C.
6.72
- D.
8
Attempted by 30 students.
Show answer & explanation
Correct answer: B
Key calculation: compute the transmission time of the minimum Ethernet frame and then adjust for both interfaces.
Step 1: Convert the minimum frame size to bits.
Minimum frame = 84 bytes = 84 × 8 = 672 bits
Step 2: Compute transmit time at 100 Mb/s.
Transmit time for one minimum frame = 672 bits / 100,000,000 bits/s = 6.72 microseconds
Step 3: Account for two full‑duplex interfaces.
To sustain wirespeed without queuing when packets may arrive on both interfaces concurrently, the router must process each packet in at most half the single-frame transmit time.
Maximum processing time per packet = 6.72 μs / 2 = 3.36 microseconds
Answer: 3.36 microseconds